When UV-irradiated T4 bacteriophage (v+) infects in the presence of chloramphenicol, the phage DNA rapidly acquires single-stranded breaks proportional to the dose of UV. In contrast, when UV-irradiated T4 v1 (radiation sensitive mutant) infects under identical conditions, the phage DNA remains integral. A series of coinfections with v+ and v1 phage (UV-v1 + majority non-UV-v+ and UV-v+ and majority non-UV-v1) show that the enzyme responsible for breakage is injected by the phage. It is also demonstrated that the v1 phage injects an inactive enzyme that delays breakage by the v+ enzyme and interferes with subsequent repair. The cross of v+ and v1 phage produces mixed progeny that contain both active and inactive enzyme in a single capsid. The possible function of this breaking enzyme, necessitating injection of multiple copies, is considered.
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